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Article

Construction of a Novel Ternary GQDs/g-C3N4/ZIF-67 Photocatalyst for Enhanced Photocatalytic Carbon Dioxide Reduction

by
Zhiyuan Zhao
1,2,
Jingjing Wang
3,
Congnian Xu
1,
Zhao Du
1,2,4,
Rongrong Yu
1,2,
Yongqi Zhao
1,2,
Jiayi Han
1,
Jingtao Zuo
1,
Zhonglu Guo
1,2,
Chengchun Tang
1,2 and
Yi Fang
1,2,*
1
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China
2
Hebei Key Laboratory of Boron Nitride Micro and Nano Materials, Hebei University of Technology, Tianjin 300130, China
3
School of Energy and Chemical Engineering, Tianjin Renai College, Tianjin 301636, China
4
Henan Province Key Laboratory of Intelligent Lighting, Huanghuai University, Zhumadian 463000, China
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(6), 334; https://doi.org/10.3390/catal14060334
Submission received: 17 April 2024 / Revised: 11 May 2024 / Accepted: 16 May 2024 / Published: 21 May 2024
(This article belongs to the Section Photocatalysis)
Browse Figures
Versions Notes

Abstract

:
In this study, graphene quantum dots (GQDs) have been incorporated into the g-C3N4/ZIF-67 heterojunction system as a photosensitizer to enhance photocatalytic conversion of CO2-to-CO. The GQDs are deposited onto the surface of g-C3N4/ZIF-67 using a simple water bath procedure. As expected, GQDs/g-C3N4/ZIF-67 presents outstanding performance in CO2 photoreduction. Among the GQDs/g-C3N4/ZIF-67 ternary photocatalysts, 7 GQDs-CN/ZIF-67 exhibits the best photocatalytic CO2 reduction ability with a CO yield of 51.71 μmol g−1, which is 5.05 and 1.87 times more than pristine g-C3N4 (10.24 μmol g−1) and g-C3N4/ZIF-67 (27.65 μmol g−1), respectively. This result shows that upon combination of GQDs with ZIF-67/g-C3N4, GQDs can be used as photosensitizers to improve the optical absorption capacity of the photocatalyst. Furthermore, GQDs serve as electron channels, facilitating the transport of photo-induced electrons from ZIF-67 to g-C3N4, which promotes photogenerated carrier separation efficiency. This study innovatively adds GQDs to the heterojunction and applies the prepared ternary composite to the CO2 photoreduction, which inspires a novel direction for the design of non-noble metal photocatalysts.

Graphical Abstract

1. Introduction

In recent years, due to ongoing industrialization, a significant quantity of carbon dioxide has been emitted into the atmosphere, leading to the gradual destruction of Earth’s existing ecological environment through the greenhouse effect [1,2]. Moreover, the growing energy shortage has compelled individuals to seek renewable and environmental alternatives as new sources of energy. In previous studies, researchers have explored various approaches to decrease the concentration of CO2 in the atmosphere, which mainly include the physical absorption of carbon dioxide for storage and chemical conversion into other chemicals [3,4]. Among them, a promising energy strategy is the conversion of carbon dioxide into valuable hydrocarbons through photocatalysis, which can effectively address both energy shortages and climate anomalies [5,6,7]. In a typical photocatalytic process, some of the electrons in the valence band absorb light energy and transition across the bandgap to the conduction band, resulting in electron–hole pairs. Subsequently, the photogenerated carriers have a reduction reaction with CO2 adsorbed on the catalyst surface [8,9,10]. This process requires photocatalysts to have appropriate bandgap and efficient charge separation ability. As a result, researchers are dedicated to finding photocatalysts that are suitable for CO2 photoreduction [11,12,13,14].
With the in-depth research on photocatalysts, graphite-phase carbon nitride (g-C3N4), a metal-free and narrow bandgap semiconductor, has received increasing attention. g-C3N4 has found extensive applications in photocatalytic hydrogen evolution, degradation, and reduction of CO2 due to its visible-light activity, low synthesis cost, environmental friendliness, and exceptional thermal and chemical stability [15]. Nonetheless, the inadequate separation efficiency of photocarriers and the limited utilization of visible-light in pure g-C3N4 limit its application in the field of photocatalysis [16,17]. To address these challenges, various strategies have been used to regulate the structure and composition of g-C3N4-based photocatalysts, such as morphology tuning [18], doping [19], and metal loading [20]. Furthermore, the formation of a heterojunction promotes the directional movement of electrons and holes, reducing the carrier-transport activation energy, which improves the photocatalytic performance of the photocatalyst [21,22]. In addition, recent advances have demonstrated that the hierarchical heterojunction with quantum dot sensitization can effectively enhance light utilization and accelerate the directional transfer of electrons [23,24]. Graphene quantum dots (GQDs) are novel carbon nanomaterials characterized by high chemical stability, excellent photoelectric properties, and efficient electron transfer/transmission capabilities. These characteristics make GQD-sensitized heterostructure materials with remarkable charge separation capabilities [25]. Several recent studies have reported heterostructures with GQDs, such as Bi2MoO6/GQDs/MoS2 [26], Ag3PO4@N-GQDs@g-C3N4 [27], and others [28,29,30], which have shown excellent photocatalytic and photoelectrochemical activities. Finally, metal organic frameworks (MOFs) formed through connecting inorganic and organic units by strong chemical bonds have emerged as an innovative catalyst system in recent years. MOFs offer advantages such as high specific surface area, abundant active sites, ease of recovery, and low toxicity [31]. ZIF-67, a MOF material featuring a zeolite imidazole salt framework, serves as an excellent heterogeneous co-catalyst due to its abundant metal ions and pores that facilitate CO2 adsorption and electron transfer [32].
In this study, it is considered that the incorporation of GQDs as a photosensitive agent into the g-C3N4/ZIF-67 heterostructure is beneficial to enhance the photocatalytic CO2 reduction ability. Therefore, a series of GQDs/g-C3N4/ZIF-67 ternary composites were prepared, which exhibited significant photocatalytic CO2 reduction activity. Based on our study, the enhancement of the photocatalytic ability by GQDs play an important role, not only as a photosensitizer to enhance the light utilization efficiency of the catalyst, but also as an electron channel to accelerate the segregation of photogenerated electrons and holes.

2. Results and Discussion

The crystal structure and micromorphology of the GQDs were characterized using XRD and TEM. From Figure 1a, it is obvious that a broad peak of diffraction appears at 2θ = 25.6°, which corresponds to the (002) crystal plane of graphene. The lattice spacing corresponding to the (002) crystal plane of the GQDs is calculated to be 0.347 nm according to the Bragg equation. Figure 1b shows the TEM image of the GQDs, which shows the morphology of the GQDs as a nanoparticle-like structure. From the HR-TEM image (inset), the lattice stripes of the GQDs can be clearly seen, and the lattice spacing of the GQDs was obtained from the measurements to be 0.35 nm, which belongs to the (002) crystalline plane, similar to the calculated results of XRD. The particle size distribution of the GQDs was fitted using the Gaussian function, and the results are shown in Figure 1c. The average size of the GQDs was calculated to be about 8.74 nm, and the fitted curve has a narrow half-peak width, indicating that the particle size distribution of the GQDs is more uniform.
The XRD pattern was recorded to characterize the crystal structures of the photocatalysts. As shown in Figure 2a, the peaks of g-C3N4 are located at 13.3° and 26.7°, which are ascribed to the in-plane repeating units of the triazine motif ((100) plane) and the graphitic interplanar packing of conjugated aromatic systems of g-C3N4 ((002) plane), respectively [33]. The presence of a highly crystalline material is indicated by the prominent peaks displayed in the XRD analysis of ZIF-67 [34,35,36]. In addition, g-C3N4/ZIF-67 presents all characteristic peaks of g-C3N4 and ZIF-67, confirming the successful formation of the composite. Notably, the XRD pattern of 7 GQD-CN/ZIF-67 does not exhibit identifiable diffraction peaks from the GQDs, likely due to their low content within the composite system. No other impurity peaks in the XRD pattern of 7 GQDs-CN/ZIF-67 indicates that the prepared composite is highly pure.
Figure 2b displays the FT-IR patterns of photocatalysts. For ZIF-67 and its composites, the bands around 600–1500 cm−1 belong to the stretching and bending vibration peaks of imidazole rings, which are typical characteristic peaks of ZIF-67 [37]. The peak located at 1580 cm−1 is the tensile vibration peak of the C=N bond. The bands observed at 2929 and 3135 cm−1 are derived from the stretching vibrations of the C-H bond in the aromatic ring [38]. Moreover, the bands between 1000 and 1700 cm−1 of each sample were related to the unique tensile vibration of the C-N heterocyclic ring. For the FT-IR pattern of g-C3N4 and its compound materials, the peak near 811 cm−1 is observed, which ascribes to the typical respiratory pattern for triazine rings [39,40]. In addition, no absorption peaks of oxygen-containing groups associated with GQDs are observed in the FT-IR of 7 GQD-CN/ZIF-67. The reason is that the characteristic peaks of C-O-C and other oxygen groups in GQDs were covered by some characteristic peaks of C-N heterocyclic rings. However, the increase of peak intensity near 3500 cm−1 in the 7 GQD-CN/ZIF-67 FT-IR pattern belongs to the presence of O-containing functional groups on the GQDs due to incomplete carbonization during the pyrolysis of citric acid.
The morphology and microstructure of the typical photocatalysts were characterized using SEM and TEM (Figure 3 and Figure 4). In Figure 3a, ultrathin nanosheets of pure g-C3N4 display an aggregation structure with inhomogeneous folds, which enhances the surface area of g-C3N4 and provides lots of surface reaction sites, thus facilitating photocatalytic activity [41]. Figure 3b shows the SEM image of ZIF-67, revealing uniform rhombic dodecahedron crystals with sharp edges [42]. Furthermore, Figure 3c shows that ZIF-67 and g-C3N4 are indeed combined while preserving the g-C3N4 microstructure. Notably, the SEM image of 7 GQDs-CN/ZIF-67 is revealed in Figure 3d, where it can be found that the morphology of g-C3N4 and ZIF-67 is not obviously changed after adding GQDs. However, due to their small size, the quantum dots cannot be detected using SEM. Therefore, the distribution of GQDs on the catalyst’s surface is further analyzed by HR-TEM.
The TEM images in Figure 4a show the g-C3N4 nanosheet, which appears to have a thin flake structure. Figure 4b displays the TEM image of 7 GQDs-CN/ZIF-67, which reveals that ZIF-67 is attached to the g-C3N4 nanoflake. Additionally, the HR-TEM images in Figure 4c,d present that the GQDs are uniformly deposited on the ZIF-67 and g-C3N4 surfaces, and the lattice fringes of the GQDs and g-C3N4 can be observed. It can be viewed from Figure 4d that the GQDs tend to have an inhomogeneous distribution and attach to the g-C3N4 surface to form a dense black spot region. In addition, the lattice spacing of the GQDs is 0.35 nm, which corresponds to the (002) crystal plane, and the lattice spacing of g-C3N4 is 0.32 nm, which corresponds to the (002) crystal plane [43,44]. Furthermore, the GQDs attached to ZIF-67 and g-C3N4 also facilitate charge transfer during the photocatalytic process. Finally, the chemical composition of 7 GQDs-CN/ZIF-67 is determined by element mapping, which reveals that it consists of C, N, O, and Co.
In order to further investigate the chemical states and related chemical bonds of the elements, XPS spectra were measured. The comprehensive patterns of g-C3N4, g-C3N4/ZIF-67, and 7 GQDs-CN/ZIF-67 are presented in Figure 5a. Both g-C3N4/ZIF-67 and 7 GQDs-CN/ZIF-67 mainly consist of C, N, O, and Co. The N 1s spectra are illustrated in Figure 5b, three peaks are observed at 398.5, 399.0, and 400.9 eV ascribed to the sp2-bonded N (C-N=C), and the tertiary N in N-(C)3 groups and amino groups (C-N-H), respectively [45,46]. Figure 5c shows the O 1s spectrum of each catalyst. The peaks at 533.2 and 531.6 eV in g-C3N4/ZIF-67 are attributed to the OH group and its lattice O, respectively. With the introduction of GQDs, the O 1s orbitals of 7 GQDs-CN/ZIF-67 do not exhibit significant changes. However, the binding energies of the two peaks are slightly shifted (533.1 and 531.5 eV). Both g-C3N4/ZIF-67 and 7 GQDs-CN/ZIF-67 exhibit similar Co 2p spectra (Figure 5d). The two main peaks of Co2+ are at 781.1 eV and 796.6 eV, corresponding to Co 2p3/2 and Co 2p1/2, respectively, while the two satellite peaks of Co 2p are situated at 785.3 eV and 802.0 eV [47]. For C 1s, spectra (Figure 5e) are detected for pure g-C3N4 and g-C3N4/ZIF-67, the peaks at 284.8, 286.2 and 288.0 eV belong to the sp2 C=C bond, the sp2 C=N bonds, and N=C-N coordination, respectively [48]. Notably, the satellite peak that appears on the 289.1 eV for 7 GQD-CN/ZIF-67 belongs to the C=O bond in the GQDs. Furthermore, g-C3N4/ZIF-67 and 7 GQDs-CN/ZIF-67 exhibit higher peak intensities at 286.2 eV compared to g-C3N4, indicating the presence of interactions in the recombination process [49].
The N2 adsorption–desorption isotherm and pore size distribution curves (Figure 6) are utilized to determine the specific surface area and pore size of each photocatalyst. Among them, g-C3N4 and 7 GQDs-CN/ZIF-67 have type IV isotherms of H3 hysteresis loops (Figure 6a,c), indicating the presence of mainly mesoporous structures in both of them. On the other hand, g-C3N4/ZIF-67 shows type IV isotherm of H4 hysteresis loop (Figure 6b), indicating the presence of microporous and mesoporous structures. From Figure 6d, the adsorption and desorption isotherms of ZIF-67 can be analyzed as type I hysteresis lines, which proves that ZIF-67 is a microporous structure, and the pore size distribution of ZIF-67 also further concludes the microporous characteristics of ZIF-67. On the basis of these calculations, the specific surface area of pure g-C3N4 is determined to be 37.10 m2 g−1, whereas ZIF-67 has a significantly larger specific surface area (1742.47 m2 g−1). When g-C3N4 is composited with ZIF-67, the BET surface area of the g-C3N4/ZIF-67 (221.29 m2 g−1) increases compared to g-C3N4. However, in the case of GQDs loading on g-C3N4/ZIF-67, the specific surface area decreases to 57.36 m2 g−1. The decrease can be attributed to the GQDs clogging some pores of g-C3N4 and ZIF-67, leading to a reduction in specific surface area and pore volume. Furthermore, ZIF-67 has a highly sensitive surface, and the immobilization of GQDs on the ZIF-67 can cause surface deformation and aggregation of new components, which also leads to a remarkable reduction in specific surface area [50]. Table 1 presents a summary of the specific surface area, pore volume, and pore size of the synthesized photocatalysts.
The light absorption of pure g-C3N4 and its composites was analyzed by UV–vis spectroscopy. As shown in Figure 7a, the bare g-C3N4 exhibits poor light absorption ability for visible light (~460 nm), while ZIF-67 displays strong absorption bands in the range of 420 to 700 nm. For g-C3N4/ZIF-67 composites, the absorption peak in the visible-light range is wider and stronger compared to g-C3N4. Importantly, 7GQDs-CN/ZIF-67 has a stronger light absorption peak over the entire wavelength range, which can be attributed to the photosensitive effect of the embedded GQDs [51]. However, when the content of GQDs reached 9 mL, a decrease in the light absorption performance was observed, which was caused by the shielding effect that made the excess GQDs hinder the light absorption of the catalysts. In addition, the bandgap energy (Eg) of g-C3N4 and ZIF-67 were calculated by the Tauc diagram of (αhν)2 versus hν (Figure 7b). The obtained bandgaps of g-C3N4 and ZIF-67 are 2.78 and 1.94 eV, respectively [52]. Furthermore, the slopes of Mott–Schottky curves of g-C3N4 and ZIF-67 are positive (Figure 7c), proving both are N-type semiconductors. The flat-band potentials (Efb) of g-C3N4 and ZIF-67 relative to Ag/AgCl electrodes are −1.41 and −1.50 eV, respectively. After conversion to ordinary hydrogen electrodes, the flat-band potentials are found to be −1.21 and −1.30 eV, respectively. Since the conduction band potential (ECB) of N-type semiconductors is 0.2 eV lower than the Efb, the ECB of g-C3N4 and ZIF-67 are −1.41 and −1.50 eV. Based on the bandgap value equation (EVB = Eg + ECB), the EVB of g-C3N4 and ZIF-67 are calculated to be 1.37 and 0.47 eV. The band structure diagram of g-C3N4 and ZIF-67 is shown in Figure 7d.
The photocatalytic performance of different photocatalysts was characterized by the photoreduction of CO2 into CO and CH4 under visible light irradiation without any sacrificial agent. As can be found in Figure 8a, the bare g-C3N4 demonstrates certain photocatalytic CO2 reduction properties, resulting in a CO production of 10.24 μmol g−1, whereas the CO production of pure ZIF-67 is only 11.56 μmol g−1 due to the poor separation of photogenerated carriers. However, when g-C3N4 is combined with ZIF-67, the CO yield reaches 27.65 μmol g−1, which is 2.70 times superior to pure g-C3N4. It indicates that the heterojunction formed by g-C3N4 and ZIF-67 inhibits the recombination efficiency of photo-induced carriers, thereby enhancing the CO2 photoreduction performance. Notably, after loading GQDs on g-C3N4/ZIF-67, the photocatalytic carbon dioxide reduction ability of the photocatalyst appears to be obviously improved. Among them, the highest photocatalytic capacity is demonstrated by 7 GQDs-CN/ZIF-67, with the CO yield reaching 51.71 μmol g−1, which is 5.05 and 1.87 times higher than bare g-C3N4 and g-C3N4/ZIF-67, respectively. However, the photocatalytic performance of the ternary composite decreases with the further increase in GQD content. This phenomenon can be attributed to excessive GQDs, which not only hinder the absorption of light by g-C3N4 and ZIF-67 due to the existing shielding effect but clog the CO2 adsorption site of ZIF-67. The great catalytic performance of 7 GQDs-CN/ZIF-67 was confirmed by comparison with other reported catalysts (Table 2). 7 GQDs-CN/ZIF-67 is not only a non-precious metal catalyst and environmentally friendly, but also exhibits the best performance in photocatalytic reduction of CO2. Figure 8b presents the curves depicting the cumulative yields of CO and CH4 over time for 7 GQDs-CN/ZIF-67. It can be observed that the yields of CO and CH4 exhibit a linear correlation with time within 6 h, suggesting the stable and persistent catalytic performance of 7 GQDs-CN/ZIF-67. Finally, to further confirm the source of CO and CH4, a series of controlled trials were conducted (Figure 8c). The results indicate that there is no gas phase product without catalyst or light, while only trace amounts of CO and CH4 are observed in the absence of H2O or CO2. Finally, photostability is also an important factor in evaluating the photocatalysts. Therefore, catalytic experiments were performed on the 7 GQDs-CN/ZIF-67 ternary photocatalyst for five cycles. As shown in Figure 8d, the CO yield does not decrease significantly after five cycles, indicating that the 7 GQDs-CN/ZIF-67 composite has great stability. In conclusion, g-C3N4/ZIF-67 sensitized by GQDs exhibits an excellent CO2 photoreduction performance.
The electron transfer of photocatalysts can be measured by PL emission spectra. Figure 9a shows the PL emission spectra of g-C3N4, g-C3N4/ZIF-67, and 7 GQDs-CN/ZIF-67 at the excitation wavelength of 370 nm. As shown in Figure 9a, g-C3N4 exhibits the highest PL emission peak, indicating the short photo-induced carrier lifetime. In addition, when g-C3N4 is combined with ZIF-67, the PL intensity is significantly reduced, which can be assigned to the heterojunction consisting of g-C3N4 and ZIF-67 promoting the effectively photo-induced charge separation. Meanwhile, the emission peak value of 7GQDs-CN/ZIF-67 is further reduced than g-C3N4/ZIF-67, indicating that the addition of GQDs promotes the charge transfer between g-C3N4 and ZIF-67. Furthermore, photoelectrochemical tests were conducted to further demonstrate that the presence of GQDs promotes photoelectron transport from g-C3N4 to ZIF-67. Figure 9b displays the photocurrent response curves of catalysts under visible light illumination. Apparently, the photocurrent intensity of 7 GQDs-CN/ZIF-67 is significantly higher than pure g-C3N4 and g-C3N4/ZIF-67, indicating a higher current density and greater photogenerated load separation efficiency. Furthermore, Figure 9c illustrates the equivalent circuit of the Nyquist plots. A lower diameter of the Nyquist plot in the electrochemical impedance (EIS) curve means a smaller charge transfer resistance. As expected, 7 GQDs-CN/ZIF-67 exhibits the smallest diameter, indicating a faster charge transfer rate. Finally, as depicted in Figure 9b,c, both pure g-C3N4 and ZIF-67 exhibit poor photogenerated carrier separation ability, resulting in the inability to generate sufficient photogenerated electrons to participate in the photocatalytic reduction of CO2. In summary, GQDs, as an ideal photosensitizer material, utilize their photosensitivity effect to establish an electron channel between g-C3N4 and ZIF-67 to promote electron transport, thus improving the photo-induced carrier separation efficiency and ultimately enhancing the CO2 photoreduction efficiency for the catalyst.
The possible photocatalytic mechanism of the GQDs/g-C3N4/ZIF-67 ternary photocatalyst is displayed in Figure 10. GQDs incorporated on the surface of g-C3N4 and ZIF-67 perform multiple roles. Firstly, GQDs act as photosensitizers to improve the optical absorptivity of the photocatalyst. Moreover, GQDs serve as an electron channel to accelerate the movement of photogenerated electrons in the ZIF-67 conduction band to the g-C3N4 conduction band, and owing to the high electron affinity of GQDs, the photogenerated electrons can continue to transfer to the GQDs located on the surface of g-C3N4. To sum up, the remarkable CO2 photoreduction efficiency of GQDs/g-C3N4/ZIF-67 can be attributed to the heterojunction formed by g-C3N4 and ZIF-67 to enhance charge separation. In addition, GQDs act as photosensitizers to improve the light utilization of the catalysts. Finally, GQDs function as electron channels to further facilitate charge transfer.

3. Experimental Section

3.1. Materials

Urea (H2NCONH2, 99%, Fengchuan Chemical Reagent Technology Co., Ltd., Tianjin China), cobalt nitrate hexahydrate (Co (NO3)2∙6H2O, 99%, Macklin Biochemical Co., Ltd., Shanghai, China), 2-methylimidazole (C4H6N2, 99%, Fengchuan Chemical Reagent Technology Co., Ltd.), sodium hydroxide (NaOH, 97%, Kermel Chemical Reagent Co., Ltd., Tianjin, China), and citric acid (C6H8O7, 99%, Kermel Chemical Reagent Co., Ltd.).

3.2. Photocatalyst Construction

3.2.1. Synthesis of g-C3N4

In this work, g-C3N4 was fabricated by directly calcinating urea powder in a Muffle furnace at 550 °C (5 °C min) for 4 h [52]. The resulting yellow g-C3N4 powder was then milled before being used for the next step.

3.2.2. Synthesis of GQDs

The citric acid cracking process was carried out to prepare GQDs. Initially, 2 g of citric acid was added into the glass container and heated directly at 200 °C for 20 min using a heating mantle. Subsequently, the obtained brown liquid was added dropwise into a continuously stirred solution of 100 mL NaOH (0.5 M), resulting in a reddish brown alkaline GQD solution. The GQD suspension was then dialyzed for 36 h and the obtained neutral solution was freeze-dried to obtain GQD powder. Finally, the GQD powder was redispersed in a specific quantity of ultrapure water to achieve a known concentration of GQD solution (2 mg mL−1).

3.2.3. Synthesis of g-C3N4/ZIF-67 Sample

The g-C3N4/ZIF-67 composite was prepared by a solvothermal process. Firstly, 0.1 g of synthesized g-C3N4 and 0.2 g of Co(NO3)2·6H2O were dispersed in 50 mL of methanol and stirred for 0.5 h to form a uniform suspension. Afterwards, 0.22 g of C4H6N2 was dissolved in 20 mL methanol, then quickly poured into the dispersion and stirred for 1 h. The dispersion was subsequently placed in a 100 mL Teflon-lined autoclave and heated in an oven at 140 °C for 12 h. The resulting liquid product was washed 3 times with methanol by centrifugation to obtain a purple precipitate. Finally, the g-C3N4/ZIF-67 powder was obtained after drying at 65 °C for 12 h.

3.2.4. Synthesis of GQDs/g-C3N4/ZIF-67 Sample

The synthetic route of the GQDs/g-C3N4/ZIF-67 ternary composites is illustrated in Figure 11. Initially, 0.1 g of the previously prepared g-C3N4/ZIF-67 was dispersed in 60mL of methanol and stirred for 1 h to obtain a homogeneous suspension. Subsequently, 3, 5, 7, and 9 mL of GQDs was added to the above solution in a water bath at 70 °C, and the methanol was completely evaporated over 8 h of stirring. Finally, the resulting solid samples were further dried at 70 °C. The ternary composites were named as 3 GQDs-CN/ZIF-67, 5 GQDs-CN/ZIF-67, 7 GQDs-CN/ZIF-67, and 9 GQDs-CN/ZIF-67 according to the GQD loading.

3.3. Catalysts Characterization

The crystal structure and physical phase of the catalysts were characterized by X-ray diffraction (XRD, D8-advance, Bruker, Billerica, MA, USA) using Cu Kα radiation at the voltage of 40 kV. The surface functional groups and chemical bonds of composites were determined using Fourier transform infrared (FT-IR, TENSOR 27, Bruker, Billerica, MA, USA) with a scanning range of 400−4000 cm−1. Scanning electron microscopy (SEM, Quanta 450FEG, FEI, Hillsboro, OR, USA) and transmission electron microscopy (TEM, JEM-F200, JEOL, Tokyo, Japan) were used to characterize the morphology, microstructure, and element distribution of the materials. The elemental composition and functional groups of the samples were analyzed by X-ray photoelectron spectra (XPS, K-Alpha, Thermo Scientific, Waltham, MA, USA). Automatic surface area and porosity analyzer (ASAP 2460, Micromeritics, Norcross, GA, USA) was applied to obtain the N2 adsorption–desorption isotherms of the samples. The UV–visible spectrophotometer (U3900H, 240–800 nm, HITACHI, Tokyo, Japan) was utilized to obtain UV–visible diffuse reflectance spectrum (UV–vis DRS). Additionally, we obtained the photoluminescence (PL) spectra using a fluorescence spectrophotometer (F-4500, HITACHI, Tokyo, Japan).

3.4. Photocatalytic Reaction

The photocatalytic CO2 reduction performance test of each sample was conducted under visible light. The entire reaction occurred in a 200 mL quartz container. We placed 40 mg of the powder sample evenly at the bottom of the container. For the purpose of removing the air from the quartz container and establishing balance between adsorption and desorption, high-purity CO2 and H2O vapors were passed through at a flow rate of 0.5 L min−1 for 50 min. A 300 W xenon lamp (λ > 420 nm, CEL-HXF300, CEAULIGHT) with 150 mW cm−2 light intensity was used as the light source, and the photocatalytic reaction time was 6 h at room temperature. Furthermore, the cycling stability of the sample was obtained through testing the photocatalytic reduction performance of CO2 on the same sample 5 times and the reaction atmosphere was refreshed by refilling the reactor with CO2 and H2O before each cycle. Finally, the resulting gases were detected using a gas chromatograph (GC-2010 Pro, SHIMADZU, Tokyo, Japan). Furthermore, the selectivity of the photocatalysts for CO was calculated using Equation (1) [43]:
S ( C O ) = 2 R ( C O ) 2 R ( H 2 ) + 2 R ( C O ) + 8 R ( C H 4 )
where R is the yield of the product after the photocatalytic reaction.

3.5. Electrochemical Measurements

The photoelectric properties of the samples were measured using a three-electrode system on the CHI670 electrochemical workstation. All the above experiments were conducted in a 0.5 M Na2SO4 solution. In the experiment, the working electrode consisted of the ITO conductive glass coated with a material film, while the counter electrode and reference electrode were composed of a Pt electrode and a Ag/AgCl electrode, respectively. To prepare the working electrode, 10 mg of sample and 10 mg of PVDF were first combined in a mortar. Afterwards, an appropriate quantity of anhydrous ethanol was gradually incorporated into the mixture and ground into a paste. The resulting mixture was uniformly applied onto an ITO glass measuring 20 × 30 mm and subsequently dried at 60 °C for a duration of 12 h.

4. Conclusions

In summary, a series of GQDs/g-C3N4/ZIF-67 ternary photocatalysts with different GQD concentrations were prepared with the water bath method. Initially, g-C3N4 is coupled with ZIF-67 to form a heterostructure. Subsequently, GQDs are introduced to further enhance the photo-induced carrier separation efficiency and the light utilization of the photocatalyst. The DRS analysis reveals that GQDs/g-C3N4/ZIF-67 exhibit an improved capacity for absorbing visible light due to the photosensitivity of GQDs. In addition, the PL emission spectra further proves that the excellent electron transport performance of GQDs promotes the separation and transfer of photogenerated carriers within the photocatalyst. Utilizing these aforementioned advantages, the 7 GQDs-CN/ZIF-67 ternary photocatalyst exhibited outstanding photocatalytic conversion of CO2 to CO under visible light illumination (51.71 μmol g−1), surpassing the CO yield of pure g-C3N4 (10.24 μmol g−1) and g-C3N4/ZIF-67 (27.65 μmol g−1) by 7.56 times and 2.10 times, respectively. In conclusion, this study further explores the research of g-C3N4-based composites in photocatalytic carbon dioxide reduction and provides a novel direction for the design of noble metal-free photocatalysts.

Author Contributions

Z.Z.: Conceptualization, Writing—Original Draft, Writing—Review and Editing, Investigation; J.W.: Visualization; C.X.: Formal analysis; Z.D.: Data curation; R.Y.: Methodology; Y.Z.: Validation; J.H.: Validation; J.Z.: Formal analysis; Z.G.: Supervision, Project administration; C.T.: Project administration, Resources; Y.F.: Supervision, Project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the project of Central-Guide-Local Science and Technology Development Fund (The project of Science and Technology Innovation Base) (226Z3606G) and the S&T Program of Hebei (22567602H).

Data Availability Statement

The data presented in this study are available on request from the corresponding author via e-mail: [email protected] (Yi Fang).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) XRD, (b) TEM, and (c) particle size distribution curve of GQDs.
Figure 1. (a) XRD, (b) TEM, and (c) particle size distribution curve of GQDs.
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Figure 2. (a) XRD patterns; (b) FT-IR spectra of g-C3N4, ZIF-67, g-C3N4/ZIF-67, and 7 GQDs-CN/ZIF-67.
Figure 2. (a) XRD patterns; (b) FT-IR spectra of g-C3N4, ZIF-67, g-C3N4/ZIF-67, and 7 GQDs-CN/ZIF-67.
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Figure 3. SEM images of (a) g-C3N4, (b) ZIF-67, (c) g-C3N4/ZIF-67, and (d) 7 GQDs-CN/ZIF-67.
Figure 3. SEM images of (a) g-C3N4, (b) ZIF-67, (c) g-C3N4/ZIF-67, and (d) 7 GQDs-CN/ZIF-67.
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Figure 4. TEM images of (a) g-C3N4, (b) 7 GQD-CN/ZIF-67; (c,d) HR-TEM; and (e–h) element mappings of 7 GQD-CN/ZIF-67.
Figure 4. TEM images of (a) g-C3N4, (b) 7 GQD-CN/ZIF-67; (c,d) HR-TEM; and (e–h) element mappings of 7 GQD-CN/ZIF-67.
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Figure 5. (a) XPS survey spectra of g-C3N4, g-C3N4/ZIF-67, and 7 GQDs-CN/ZIF-67. (b) N 1s, (c) O 1s, (d) Co 2p, and (e) C 1s high-resolution XPS spectra, respectively.
Figure 5. (a) XPS survey spectra of g-C3N4, g-C3N4/ZIF-67, and 7 GQDs-CN/ZIF-67. (b) N 1s, (c) O 1s, (d) Co 2p, and (e) C 1s high-resolution XPS spectra, respectively.
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Figure 6. N2 adsorption-desorption isotherms and pore size distribution curve (inset) results of (a) g-C3N4, (b) g-C3N4/ZIF-67, (c) 7 GQDs-CN/ZIF-67, and (d) ZIF-67.
Figure 6. N2 adsorption-desorption isotherms and pore size distribution curve (inset) results of (a) g-C3N4, (b) g-C3N4/ZIF-67, (c) 7 GQDs-CN/ZIF-67, and (d) ZIF-67.
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Figure 7. (a) UV-vis curves of g-C3N4, ZIF-67, g-C3N4/ZIF-67, and a series of GQDs/g-C3N4/ZIF-67; (b) Tauc plots of g-C3N4 and ZIF-67 were used to calculate the bandgap energies; (c) Mott-Schottky curves of g-C3N4 and ZIF-67; (d) an energy band structure schematic.
Figure 7. (a) UV-vis curves of g-C3N4, ZIF-67, g-C3N4/ZIF-67, and a series of GQDs/g-C3N4/ZIF-67; (b) Tauc plots of g-C3N4 and ZIF-67 were used to calculate the bandgap energies; (c) Mott-Schottky curves of g-C3N4 and ZIF-67; (d) an energy band structure schematic.
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Figure 8. (a) A comparison of total products for g-C3N4, ZIF-67, g-C3N4/ZIF-67, and a series of GQDs/g-C3N4/ZIF-67 composites after 6 h light irradiation; (b) the typical time courses of CO and CH4 evolutions; (c) the evolution of CO and CH4 under various reaction conditions; and (d) the formation of CH4 and CO in stability tests of 7 GQDs-CN/ZIF-67.
Figure 8. (a) A comparison of total products for g-C3N4, ZIF-67, g-C3N4/ZIF-67, and a series of GQDs/g-C3N4/ZIF-67 composites after 6 h light irradiation; (b) the typical time courses of CO and CH4 evolutions; (c) the evolution of CO and CH4 under various reaction conditions; and (d) the formation of CH4 and CO in stability tests of 7 GQDs-CN/ZIF-67.
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Figure 9. (a) PL spectra, (b) transient photocurrent response, and (c) EIS of g-C3N4, ZIF-67, g-C3N4/ZIF-67, and various composites.
Figure 9. (a) PL spectra, (b) transient photocurrent response, and (c) EIS of g-C3N4, ZIF-67, g-C3N4/ZIF-67, and various composites.
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Figure 10. Schematic illustration of a possible photocatalytic mechanism for GQDs/g-C3N4/ZIF-67.
Figure 10. Schematic illustration of a possible photocatalytic mechanism for GQDs/g-C3N4/ZIF-67.
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Figure 11. Synthesis schematic illustration of GQDs/g-C3N4/ZIF-67.
Figure 11. Synthesis schematic illustration of GQDs/g-C3N4/ZIF-67.
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Table 1. Specific surface area, pore volume, and pore size of different photocatalysts.
Table 1. Specific surface area, pore volume, and pore size of different photocatalysts.
PhotocatalystSpecific Surface Area (m2 g−1)Pore Volume (cm3 g−1)Pore Size (nm)
g-C3N437.100.06819.965
ZIF-671742.470.6814.430
g-C3N4/ZIF-67221.290.16018.535
7 GQDs-CN/ZIF-6757.360.07816.791
Table 2. Comparison of performance with other reported photocatalysts for CO2 conversion.
Table 2. Comparison of performance with other reported photocatalysts for CO2 conversion.
SampleLight (nm)Time (h)CO Product (umol/g)Yields
(μmol/g/h)		Selectivity(%)Reference
7 GQDs-CN/ZIF-67λ > 420651.718.6281.06This Work
g-C3N4/Au/CeO2/Fe3O4λ > 420428.007.0042.42[53]
NiO/g-C3N4 QDs200 < λ < 900518.903.7864.80[54]
COF/g-C3N4 (Pt)λ < 40018117.236.5172.98[55]
CuOx/g-C3N4/MnOxλ > 420421.965.49none[56]
SrTiO3@NiFe LDHλ > 420647.407.9056.83[57]
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Zhao, Z.; Wang, J.; Xu, C.; Du, Z.; Yu, R.; Zhao, Y.; Han, J.; Zuo, J.; Guo, Z.; Tang, C.; et al. Construction of a Novel Ternary GQDs/g-C3N4/ZIF-67 Photocatalyst for Enhanced Photocatalytic Carbon Dioxide Reduction. Catalysts 2024, 14, 334. https://doi.org/10.3390/catal14060334

AMA Style

Zhao Z, Wang J, Xu C, Du Z, Yu R, Zhao Y, Han J, Zuo J, Guo Z, Tang C, et al. Construction of a Novel Ternary GQDs/g-C3N4/ZIF-67 Photocatalyst for Enhanced Photocatalytic Carbon Dioxide Reduction. Catalysts. 2024; 14(6):334. https://doi.org/10.3390/catal14060334

Chicago/Turabian Style

Zhao, Zhiyuan, Jingjing Wang, Congnian Xu, Zhao Du, Rongrong Yu, Yongqi Zhao, Jiayi Han, Jingtao Zuo, Zhonglu Guo, Chengchun Tang, and et al. 2024. "Construction of a Novel Ternary GQDs/g-C3N4/ZIF-67 Photocatalyst for Enhanced Photocatalytic Carbon Dioxide Reduction" Catalysts 14, no. 6: 334. https://doi.org/10.3390/catal14060334

APA Style

Zhao, Z., Wang, J., Xu, C., Du, Z., Yu, R., Zhao, Y., Han, J., Zuo, J., Guo, Z., Tang, C., & Fang, Y. (2024). Construction of a Novel Ternary GQDs/g-C3N4/ZIF-67 Photocatalyst for Enhanced Photocatalytic Carbon Dioxide Reduction. Catalysts, 14(6), 334. https://doi.org/10.3390/catal14060334

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